This invention relates to a perpendicular magnetization type magnetic recording medium, more specifically to a magnetic recording medium including a magnetic powder excellent in a perpendicular orientation ability and having remarkable improved S/N properties.
The magnetic recording medium can be manufactured by, for example, coating, in the form of a layer, a support such as a polyester film with a magnetic coating composition comprising the magnetic powder and a binder resin or comprising these components and a variety of additives, if desired; carrying out a magnetical orientation treatment for the magnetic powder; and curing the coating layer of the resin (see Japanese Patent Publication No. 10773/1983 entitled "Method for preparing magnetic recording medium").
In this case, as the magnetic powder, a needle magnetic powder has heretofore been used. In order to heighten a recording density of the magnetic recording medium, it is inevitably necessary to lessen a particle diameter of the magnetic powder particle used. However, in the case of the presently used needle magnetic powder where a shape anisotropy is utilized, it is very difficult to minimize the particle diameter of the powder particles.
Moreover, a further development in the magnetic recording is directed toward a high-quality image recording, and such development is considered to be conveniently accomplished by replacing a conventional analog recording system with a digital recording system.
The magnetic powder used in the recording medium for this digital recording system must be capable of a perpendicular magnetic recording and must be smaller in particle diameter than the conventional needle magnetic powder. Therefore, hexagonal system ferrite magnetic powders, above all, barium ferrite magnetic powders are advantageous.
With regard to the hexagonal system ferrite magnetic powder, its crystal has such a shape as a hexagonal plate and possesses a magnetic easy axis in a direction perpendicular to the plate surface.
When such a magnetic powder is employed to manufacture the recording medium, its recording density will be greatly affected by a distributive condition of the magnetic powder in the aforesaid coating layer.
Thus, when an average particle diameter of the used hexagonal ferrite magnetic powder is lessened for the purpose of heightening the recording density, it will be more difficult to disperse the magnetic powder in the state of separate particles in the binder resin because of flocculation of the magnetic powder, and it will be impossible to obtain the high-density recording which can be supposed from the particle diameter of the powder.
Further, when the magnetic coating composition is applied to the surface of the support and the magnetic easy axes are oriented in a perpendicular direction to the support, a sufficient perpendicular orientation ability cannot be obtained, if the magnetic powder has a small particle diameter. As a result, such S/N properties as are expected cannot be obtained.
This phenomenon would be considered to be due to the fact that the dispersion of the hexagonal system ferrite magnetic powder is poor or the respective magnetic powder particles are freely rotated by an interaction of these particles and the binder resin in the coating layer during the perpendicular orientation treatment in order to prevent the magnetic powder from taking a desired distribution.
Heretofore, as the binder resins, there have been often used resins plentifully including groups containing active hydrogen atoms such as a hydroxyl group, a carboxyl group, an urethane bond, an urea bond and the like (for example, see Japanese Patent Provisional Publications No. 205828/1982, No. 113420/1982 and No. 27429/1982). However, in the case that by using such a resin together with the hexagonal system ferrite magnetic powder, the magnetic coating composition is prepared, the smaller the particle diameter of the magnetic powder, the easier the gelation of the coating composition, and when allowed to stand, such a coating composition will tend to bring about the flocculation. Accordingly, in order to obtain a coating film having a smooth surface by the use of such a coating composition, it will be essential to make use of a high shearing force at a coating operation.
When the coating film which has been formed by employing the coating composition in the state of such a dispersion is placed in an orientation magnetic field, the flocculation of the magnetic powder will occur owing to the action of the magnetic field, with the result that a rough surface will be formed on the coating film. This rough surface will increase fluctuation of a recording reproduction output and will lead to noise greater than an output-improving effect based on the magnetic orientation, whereby the S/N will deteriorate.
This phenomenon would be supposed to be due to the fact that groups including active hydrogen in the resin are securely adsorbed on or bound to the surfaces of the hexagonal system ferrite magnetic powder particles, so that the distribution state of the magnetic powder in the coating composition is stably retained for a long period of time, but a kind of network structure is also formed in the coating composition as a consequence a free rotary motion or a translational motion of the respective magnetic powder particles is disturbed and masses resulting from the flocculation of the magnetic powder carry out a rotary or a translational motion, so that smoothness on the surface of the coating film is worsened.